The present invention relates to color printers wherein one or more Raster Output Scanners (ROSs) form color images on exposure frames of a photoreceptor belt and, more particularly, to a method and apparatus for forming the color images in registration.
Printers are known in the art wherein a Raster Output Scanner is positioned in an optical scan system so as to write an image on the surface of a moving photoreceptor belt. A modulated laser beam is swept across the photoreceptor surface after being directed against the facets of a rotating polygon. Each sweep exposes a raster line to a linear segment of a video signal image.
In a color xerographic ROS printer, a lurality of ROS units are positioned adjacent a photoreceptor belt surface and selectively energized to create successive image exposures until a series of image frames, each of a separate color image, is formed. For full color, four imagers are used, one for each of the three basic colors and a fourth for black images. Each image is developed in overlying registration with preceding images and a composite color image is transferred to an output sheet. Each image must be registered in both the photoreceptor process direction and in the direction perpendicular to the process direction (referred to as the fast scan or lateral direction). One registration technique is to form a plurality of holes in non-image areas of the belt at some predetermined distance upstream from an associated image exposure frame. The ROS beam is positioned so that each line sweep, or scan, begins and ends in non-image areas of the photoreceptor so as to scan across the belt holes as the belt advances into an exposure zone. Each ROS has an associated pair of sensors positioned beneath the belt. These sensors detect the ROS scanning beams as it is swept across the leading edge of a belt hole and provides output signals which are used for registration purposes. This type of ROS registration technique is generally disclosed in U.S. Pat. No. 5,302,973 issued on Apr. 12, 1994, and assigned to the same assignee as the present invention.
A major difficulty in registering the images in a ROS belt hole type detection system lies in the scanning nature of the ROS device; the spot which is swept across the photoreceptor moves at a very high rate (60,000 ips) which reduces the time the beam illuminates the photodetector through the belt hole. Since the photodetector typically has a width of only a few millimeters, the ROS beam is only sensed by the photodetector for a short time duration measured in microseconds. It has proven difficult to design detection circuits to achieve the required registration accuracy of 5-10 microns.
Several attempts have been made to design faster detection and amplification circuits. U.S. Pat. No. 5,321,434 issued on Jun. 14, 1994, and assigned to the same assignee as the present invention utilizes a split cell photodetector in conjunction with amplifying means. U.S. Pat. No. 5,278,587 issued on Jan. 11, 1994, and assigned to the same assignee as the present invention connects the photodetector output to a linear interpolation circuit to achieve a more precise method of identifying the leading edge of the holes. These prior art, co-pending applications are all hereby incorporated by reference.
There is still a need for a more accurate method of registering the color image in a ROS-type printing system. The present invention is directed to a registration technique which does not rely on the light from the ROS beam to be sensed by a photodetector. instead, a separate dedicated light source is provided that does not move in the fast scan direction, as is the case with the ROS beams sweeping across the photoreceptor. Two separate light sources, which are LEDs in a preferred embodiment, are located in the ROS optical system so as to form two light beams outside the scanning width of the ROS beams. The beams provide a continuous light flux along areas of the photoreceptor and "look" for the belt holes formed on the belt. When the belt holes are illuminated by the light sources, the photodetectors on the other side of the belt are illuminated and provide a continuous signal which simplifies the associated amplifier design and allows for more accurate sensing of the belt hole locations. More particularly, the present invention relates to an imaging system for forming multiple image exposures on exposure frames of a photoconductive member including:
a photoreceptor belt having a plurality of image exposure frames along its circumference, said belt having at least a first and second hole on opposite sides of the belt width and outside of the exposure frames,
a plurality of ROS imagers, each ROS imager associated with the formation of one of said image exposure frames, each imager forming a plurality of projected scan lines in a fast scan direction across the belt width, said scan lines beginning and ending at points within an image exposure frame,
a first and second light source opposed from one surface of the photoreceptor belt for illuminating said first and second belt holes,
first and second detecting means associated with each of said imagers, said detecting means opposed from the other surface of the photoreceptor for sensing the light from said first and second light sources through said belt holes and generating signals representing said detection, and
circuit means to amplify said detecting means output signals and to generate image registration correction signals.